1. Field of the Invention
The present invention relates to a polyisocyanate compound having a uretidione skeleton structure, a process for producing the compound and a polyisocyanate polyaddition composition comprising the compound. Since the uretidione skeleton structure of the polyaddition composition releases an isocyanate by heating, the composition is useful as the material for the curing agent for polyurethane-based thermosetting resins and can be advantageously used for powder coating materials.
2. Description of the Related Arts
It is the general situation at present that coating materials based on organic solvents are being replaced with coating materials which do not use organic solvents such as aqueous coating materials and powder coating materials due to the problems on safety and health. By the use of the powder coating materials, the environmental pollution can be decreased since the powder coating materials can be recovered and reused, and improvements in the physical properties can be expected since the powder coating materials can form thick coating films. In particular, polyurethane-based powder coating materials exhibit excellent weatherability and physical properties of the coating films and are expected to replace the coating materials based on organic solvents. The polyurethane powder coating materials are useful as the coating materials for electric appliances, building materials, automobile members, office furnitures and members of electric instruments. Precoat metals coated with the polyurethane powder coating materials can be used for electric appliances, building materials, road construction materials, automobile members, automobiles bodies, office furnitures and members of electric instruments.
As the polyurethane coating material, coating materials which use polyester polyols as the main component and curing agents obtained by blocking isocyanate group in isophorone diisocyanate (referred to as IPDI, hereinafter), which is an alicyclic diisocyanate, with a blocking agent such as ε-caprolactam are mainly used (Japanese Patent Application Publication No. Heisei 3(1991)-31744).
However, since crosslinking of the polyol is achieved by dissociation of the blocking agent during baking of the coating film in accordance with the process using the above materials, the discharged ε-caprolactam adversely affects the human health, and contamination of the oven takes place. To overcome the above problems, processes and materials which do not use blocking agents and utilize the uretidione skeleton structure such as a process for producing a uretidione compound using IPDI (Japanese Patent Application Publication No. Heisei 3(1991)-80790), a process for producing a polyaddition composition having the uretidione skeleton structure (Japanese Patent Application Publication No. Showa 64(1989)-11052) and a powder coating material using this polyaddition composition as the curing agent (Japanese Patent Application Publication No. Showa 64(1989)-5627) have been disclosed. The uretidione skeleton structure is formed by dimerization of isocyanate group and, in general, the uretidione skeleton structure and a polyol do not react with each other. Compounds having the uretidione skeleton structure are useful as the curing agent in powder coating materials since, when the coating film is heated during baking, the uretidione skeleton structure is dissociated to form two isocyanate groups, and the formed isocyanate group can react with the polyol. Since the content of the uretidione skeleton structure provides the content of NCO group during baking of the coating film, this content is defined as the content of the latent NCO group, which will be described more specifically later.
IPDI provides coating films exhibiting more excellent weatherability than that obtained by using aromatic isocyanates since IPDI is an alicyclic isocyanate. However, the curing agent based on the uretidione compound of IPDI cannot satisfy the requirements of the market due to drawbacks such that the formed powder coating film has a problem on smoothness of the surface due to a great viscosity, and that the dissociation temperature is high due to the chemical structure having asymmetric isocyanate groups, and the gel fraction cannot be sufficiently increased, resulting in insufficient mechanical properties of the coating film such as insufficient impact strength. To overcome the drawbacks of the curing agent based on the uretidione compound of IPDI, a polyester polyol or a polycarbonate polyol is used as the diol used for producing the polyaddition composition (Japanese Patent Application Laid-Open No. Heisei 7(1995)-82339). However, this process is economically disadvantageous since the content of the latent NCO group decreases, and the amount of the added curing agent increases. A polyisocyanate composition having a uretidione skeleton structure which uses hexamethylene diisocyanate (referred to as HDI, hereinafter), which is an aliphatic diisocyanate, as the material exhibits poor resistance to blocking and is not suitable for the powder coating material. A uretidione curing agent using a bis(isocyanatomethyl)cyclohexane and not using blocking agents has been disclosed (Japanese Patent Application Laid-Open No. Heisei 10(1998)-182787). However, this curing agent has a great isocyanurate fraction [the content of the isocyanurate skeleton structure×100/(the content of the isocyanurate skeleton structure+the content of the uretidione structure)] and a small content of the uretidione skeleton structure. The cost increases due to the increased amount of the curing agent, and the above agent is unsatisfactory as the curing agent.
The present invention has an object of providing a polyisocyanate compound which overcomes the drawbacks of conventional polyisocyanate compounds having the uretidione skeleton structure and polyaddition compositions containing the conventional polyisocyanate compounds, a process for producing the polyisocyanate compound and a polyisocyanate polyaddition composition. In other words, the present invention has an object of providing a polyisocyanate polyaddition composition which forms a coating film exhibiting excellent smoothness of the surface, weatherability and mechanical properties when the composition is used as the curing agent for powder coating materials.